The present disclosure is generally related to imaging members and more particularly related to photosensitive members and in embodiments to imaging members and methods for preparing same. In embodiments, a two pass process is employed to prepare an imaging member having a first charge transport layer comprising a small molecule charge transport material and a polymeric component selected from the group consisting of a polyarylamine polyester (PAPE), polyacylamine (PAA), and mixtures and combinations thereof; and a second charge transport layer disposed over the first charge transport layer, the second charge transport layer comprising a small molecule charge transport material and a binder, wherein the second charge transport layer is free of PAPE and PAA.
In the art of electrophotography, an electrophotographic plate comprising a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging the surface of the photoconductive insulating layer. The plate is then exposed to a pattern of activating electromagnetic radiation such as light, which selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic toner particles on the surface of the photoconductive insulating layer. The resulting visible toner image can be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photoconductive insulating layers.
Electrophotographic imaging members are usually multilayered photoreceptors that comprise a substrate support, an electrically conductive layer, an optional hole blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer in either a flexible belt form or a rigid drum configuration. Multilayered flexible photoreceptor belts may include an anti-curl layer on the backside of the substrate support, opposite to the side of the electrically active layers, to render the desired photoreceptor flatness. One type of multilayered photoreceptor comprises a layer of finely divided particles of a photoconductive inorganic or organic compound dispersed in an electrically insulating organic resin binder. The charge generating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer. Photoreceptors can also be single layer devices. For example, single layer organic photoreceptors typically comprise a photogenerating pigment, a thermoplastic binder, and hole and electron transport materials.
U.S. Pat. No. 4,265,990, which is hereby incorporated by reference herein in its entirety, discloses a layered photoreceptor having a separate charge generating (photogenerating) layer (CGL) and charge transport layer (CTL). The charge generating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer. The photogenerating layer utilized in multilayered photoreceptors include, for example, inorganic photoconductive particles or organic photoconductive particles dispersed in a film forming polymeric binder. Inorganic or organic photoconductive materials may be formed as a continuous, homogeneous photogenerating layer.
Examples of photosensitive members having at least two electrically operative layers including a charge generating layer and diamine containing transport layer are disclosed in U.S. Pat. Nos. 4,265,990; 4,233,384; 4,306,008; 4,299,897; and 4,439,507, the disclosures of each of which are hereby incorporated by reference herein in their entireties.
Charge transport layers are known to be comprised of any of several different types of polymer binders that have a charge transport material dispersed therein. The charge transport layer can contain an active aromatic diamine small molecule charge transport compound dissolved or molecularly dispersed in a film forming binder. This type of charge transport layer is described, for example, in U.S. Pat. No. 4,265,990, the disclosure of which is incorporated by reference herein in its entirety. Although excellent toner images can be obtained with such multilayered photoreceptors, it has been found that when high concentrations of active aromatic diamine small molecule charge transport compound are dissolved or molecularly dispersed in a film forming binder, the small molecules tend to crystallize with time under conditions such as higher machine operating temperatures, mechanical stress or exposure to chemical vapors. Such crystallization can cause undesirable changes in the electro-optical properties, such as residual potential build-up which can cause cycle-up. Moreover, the ranges of binders and binder solvent types available for use during coating operations is limited when high concentrations of the small molecules are sought for the charge transport layer.
Another type of charge transport layer has been described which uses a charge transport polymer. This type of charge transport polymer includes, but is not limited to, materials such as poly-N-vinyl carbazole, polysilylenes, and others. Other charge transporting materials include polymeric arylamine compounds and related polymers. Charge transport layer materials such as these are described in U.S. Pat. Nos. 4,801,517; 4,806,443; 4,806,444; 4,818,650; 4,871,634; 4,935,487; 4,937,165; 4,956,440; 4,959,288; 5,030,532; 5,155,200; 5,262,512; 5,306,586; 5,342,716; 5,356,743; 5,413,886; 5,639,581; 5,770,339; and 5,814,426; the disclosures of each of which are incorporated by reference herein in there entireties.
The appropriate components and process aspects of the each of the foregoing U.S. Patents may be selected for the present disclosure in embodiments thereof.
The sensitivity of a layered device depends on several factors: (1) the fraction of the light absorbed, (2) the efficiency of photogeneration within the pigment crystals, (3) the efficiency of injection of photogenerated holes into the transport layer and (4) the distance the injected carrier travels in the transport layer between the exposure and development steps. The fraction of the light absorbed can be maximized by the employment of adequate concentration of pigment in the generator layer and the selected thickness of the generator layer. The distance the carrier travels in the transport layers depends on the structure of transporting material and the binder and on the concentration of the charge transporting active molecules in the case of transport layers having a dispersion of transport active molecules in a non-transporting inactive binder. However, depending on the structure of the binder and the molecule, crystallization sets in if the concentration of the charge transport molecules is increased beyond a certain point. Including additional small molecule beyond a certain amount can result in crystallization of the material and will not lead to an increase in mobility. As more and more polymer is displaced with small molecule, the crack resistance of the entire layer is decreased. Crystallization also results in increased residuals and image defects both of which are undesirable. Therefore, the concentration limit of the charge transport molecule in the transport layer results in a limit to the speed of the electrophotographic process. If the time between exposure and development is reduced to a value that is lower than the transit time in the charge transport layer of the charge carrier injected from the generator layer, the sensitivity of the device is reduced.